InAs/InP quantum dash buried heterostructure mode-locked laser for high capacity fiber-wireless integrated 5G new radio fronthaul systems.

We have developed and experimentally demonstrated a highly coherent and low noise InP-based InAs quantum dash (QDash) buried heterostructure (BH) C-band passively mode-locked laser (MLL) with a pulse repetition rate of 25 GHz for fiber-wireless integrated fronthaul 5G new radio (NR) systems. The device features a broadband spectrum providing over 46 equally spaced highly coherent and low noise optical channels with an optical phase noise and integrated relative intensity noise (RIN) over a frequency range of 10 MHz to 20 GHz for each individual channel typically less than 466.5 kHz and -130 dB/Hz, respectively, and an average total output power of ∼50 mW per facet. Moreover, the device exhibits low RF phase noise with measured RF beat-note linewidth down to 3 kHz and estimated timing jitter between any two adjacent channels of 5.5 fs. By using this QDash BH MLL device, we have successfully demonstrated broadband optical heterodyne based radio-over-fiber (RoF) fronthaul wireless links at 5G NR in the underutilized spectrum of around 25 GHz with a total bit rate of 16-Gb/s. The device performance is experimentally evaluated in an end-to-end fiber-wireless system in real-time in terms of error vector magnitude (EVM) and bit error rate (BER) by generating, transmitting and detecting 4-Gbaud 16-QAM RF signals over 0.5-m to 2-m free-space indoor wireless channel through a total length of 25.22 km standard single mode fiber (SSMF) with EVM and BER under 8.4% and 2.9 × 10-5, respectively. The intrinsic characteristics of the device in conjunction with its system transmission performance indicate that QDash BH MLLs can be readily used in fiber-wireless integrated systems of 5G and beyond wireless communication networks.

Monolithic InAs/InP quantum dash dual-wavelength DFB laser with ultra-low noise common cavity modes for millimeter-wave applications.

We have developed and experimentally demonstrated a novel monolithic InAs/InP quantum-dash dual-wavelength distributed feedback (QD DW-DFB) C-band laser as a compact optical beat source to generate millimeter-wave (MMW) signals. The device uses a common gain medium in a single cavity structure for simultaneous correlated and stable dual-mode lasing in the 1550-nm wavelength range. A record narrow optical linewidth down to 15.83 kHz and average relative intensity noise (RIN) as low as -158.3 dB/Hz from 10 MHz to 20 GHz are experimentally demonstrated for the two optical modes generated by the laser. As a result, the beat note between these two lasing modes generates spectrally pure MMW signals between 46 GHz and 48 GHz. Such an efficient, coherent, and compact optical source is extremely attractive for applications in MMW systems, such as Radar and fiber-wireless integrated fronthaul for 5G and beyond.

Broadband LP01-LP02 mode converter for O-, E-, S-, C-, L-, and U-bands.

This paper proposes a novel all-optical fiber mode converter for mode conversion from LP01 to LP02 and vice versa. The mode converter is formed by connecting a single-mode fiber, a taper-core multi-mode fiber, and a few-mode fiber (FMF) together. The taper fiber core is designed to convert LP01 mode to LP02. The few-mode fiber is used to cut off the modes that are higher than LP02. It is shown that the proposed mode converter provides 20 dB extinction ratio and low insertion loss in a very broad optical bandwidth of 200 nm, from 1465 to 1665 nm. By further optimizing the FMF radius only for each of the O-, E-, S-, C-, U-, etc. bands, the mode converter can apply to any band of interest with higher performance. It is found that the performance of the mode converter has a large tolerance to structural parameters. The mode converter has the same performance in reciprocal operation, i.e., LP02 to LP01.

Design of modified InGaAs/InP one-sided junction photodiodes with improved response at high light intensity.

A modified InGaAs/InP one-sided junction photodiode (MOSJ-PD) is presented for the first time. The MOSJ-PD is proposed from the one-sided junction photodiode by inserting a cliff layer into the absorption layer. Compared to the modified uni-traveling carrier photodiode, the MOSJ-PD has the advantages of simpler epitaxial layer structure and lower junction capacitance. In the MOSJ-PD, the space charge effect at high light intensity can be suppressed. Thus, both 3-dB bandwidth and output current can be improved simultaneously. The performance characteristics of the MOSJ-PD, including energy band diagram, internal electric field, frequency response, photocurrent, and responsivity, are carefully studied.

Multi-objective optimization framework for designing photonic crystal sensors.

In this paper, a novel framework for designing optimized photonic crystal (PhC) sensors has been proposed. The complexity of such structures has resulted in the lack of an analytical method to design the structures. Therefore, this framework aims to provide a comprehensive and automatic method to find the best values for the structural parameters without human involvement. The framework is explained with an example of designing a PhC liquid sensor. In the framework, an optimizer called the "multi-objective gray wolf optimizer" is utilized. However, a diverse range of multi-objective optimizer algorithms could be utilized. The results show that the proposed framework can design any kind of PhC sensor. Simplicity, being straightforward, and no human involvement are the advantages of the proposed framework. In addition, a significantly wide range of optimal designs will be found that are suitable for general and specific applications.

Simple and seamless broadband optical frequency comb generation using an InAs/InP quantum dot laser.

A simple and seamless broadband optical frequency comb (OFC) generator is proposed and experimentally demonstrated using a Fabry-Perot quantum dot mode-locked laser combined with a dual-driven LiNbO3 Mach-Zehnder modulator driven by a low-power radio frequency (RF) signal. It is experimentally demonstrated that the 10-dB seamless bandwidth of the OFC is 8.2 nm (1.02 THz), which has 62 and 40 comb lines for frequency intervals of 16.56 GHz and 24.84 GHz, respectively. The single-sideband phase noise is as low as -112 and -108 dBc/Hz at an offset of 10 kHz, respectively, for the photodetector-converted 16.56 and 24.84 GHz frequency carriers. Correspondingly, the RF linewidths of the 16.56 GHz and 24.84 GHz carriers are about 251 Hz-263 Hz, respectively. Using a QD laser, an ultra-low phase noise and quasi-tunable broadband OFC generator is obtained easily.

Analysis of optical fiber-based LP(01) ↔ LP(02) mode converters for the O-, S-, and C-Band.

This paper proposes a double fiber taper-based mode converter that converts LP(01) to LP(02) and vice versa. The first taper is used to convert LP(01) to some higher order LP(0m) (m>1) modes while the second taper suppresses the undesired higher order modes (m>2) and results in LP(02) overwhelmingly dominated. A simulation shows that conversion efficiency of almost 100% at the central wavelength of O-, S-, and C-Band, and above 98% over the S- and C-Band was achieved. Moreover, suppression of undesired higher order modes is more than 10 dB over the whole O-, S-, and C-Band. In particular, the suppression is more than 19 dB for the whole C-Band. The analysis also shows that the performance of the mode converter is not sensitive to slight variations of the converter's parameters. The same converter also can be used to convert LP(02) back to LP(01). It is shown that conversion efficiency higher than 99% and suppression of undesired higher order modes higher than 30 dB can be obtained. Further, a (de)multiplexer for an LP(02) and an LP(01) mode was designed using the mode converter combined with a symmetric directional coupler. The multiplexer is broadband and has an insertion loss less than 0.5 dB in the C-Band.

Design of broadband and high-output power uni-traveling-carrier photodiodes.

In this paper, physically-based simulations are carried out to investigate and design broadband and high-output power uni-traveling carrier (UTC) photodiodes. The physical model is first verified by comparison to experimentally measured results. The graded-bandgap structure, which can induce potential gradient, is considered to be used in the absorption layers. It is shown that the electric field in the absorption layer is increased by the gradient, thus the performance of bandwidth and saturation current is improved by 36.6% and 40% respectively for our considered photodiode. Moreover, a modified graded-bandgap structure is proposed to further increase the electric field, and an additional 9.5% improvement in bandwidth is achieved. The final proposed UTC-PD structures will result in 399-GHz bandwidth and 49-mA DC saturation current.

Linearized optical single sideband Mach-Zehnder electro-optic modulator for radio over fiber systems.

We propose a linearized optical single sideband Mach-Zehnder electro-optic modulator (MZ-EOM) for radio over fiber systems. This proposed modulator utilizes a dual-electrode MZ-EOM biased at quandrature, the two electrodes are fed with asymmetrical RF power through an 180? hybrid coupler and the two optical ports of MZ-EOM are bi-directionally injected by the same optical source, but with different optical power. Using such a modulator, third order nonlinear distortion is significantly suppressed and relative spurious free dynamic range is improved by 10 and 20 dB compared to dual-parallel linearized MZ-EOM and conventional MZ-EOM, respectively.

Frequency sixupler for millimeter-wave over fiber systems.

In this work, we propose and investigate a novel technique for the generation of millimeter-wave (mm-wave), i.e. frequency sixuplexing technique. The proposed technique is comprised of two cascaded Mach- Zehnder modulators (MZMs). The first MZM, biased at maximum transmission, is only used for even-order optical harmonic generation, and then a second MZM, biased at minimum transmission, is used for both optical carrier suppression modulation and data signal modulation. As an example, we consider an RF at 10 GHz, which carries the data signal and drives the MZMs; and an mm-wave signal at 60 GHz, i.e. a frequency sixupler, is obtained. It is found that our proposed sixupler leads to an 8-dB higher RF power at 60 GHz and a 6-dB improvement in receiver sensitivity with comparison to the conventional technique, i.e. optical carrier suppression modulation. The generated mm-wave signal is robust to fiber chromatic dispersion. The proposed technique is verified by experiments.

Analysis of frequency quadrupling using a single Mach-Zehnder modulator for millimeter-wave generation and distribution over fiber systems.

We comprehensively investigate three modulation techniques for the generation of millimeter-wave (mm-wave) using optical frequency quadrupling with a dual???electrode Mach-Zehnder modulator (MZM), i.e. Technique-A, Technique-B and Technique-C. For Technique-A, an RF signal drives the two electrodes of the MZM with maximum transmission bias, and this MZM is used for both the mm-wave generation and signal modulation. Technique-B is the same as Technique-A, but 180(0) phase shift between the two electrodes is applied. Technique-C is the same as Technique-B, but the MZM is only used for the mm-wave generation without signal modulation. It is found that Technique-B and Technique-C are better for frequency quadrupling than frequency doubling, tripling and sextupling. Both theoretical analysis and simulation show that the generated mm-wave suffers from constructive/destructive interaction due to fiber chromatic dispersion in Technique-A. However, the generated mm-wave is almost robust to fiber chromatic dispersion in Technique-B and Technique- C. It is found that Technique-C is the best in the quality of the generated mm-wave, especially when poor optical filtering is used. In addition, we develop a theory for calculation of Q-factor for mm-wave generation using the three modulation techniques. We consider an RF at 7.5 GHz and obtain an mm-wave at 30 GHz as an example, i.e. a frequency quadrupler. We evaluate the generation and distribution in terms of system Q-factor. The impact of RF modulation index, chromatic dispersion, MZM extinction ratio and optical filtering on Q-factor are investigated.

Frequency upconversion of multiple RF signals using optical carrier suppression for radio over fiber downlinks.

We propose and analyze a technique of an optical carrier transmitting two RF signals using optical carrier suppression. A single optical Mach-Zehnder modulator is used for both optical carrier suppression and signal modulation, and optical carrier suppression modulation is also used for frequency conversion of RF signals. This work shows that in contrary to the case of an optical carrier transmitting a single RF signal with optical carrier suppression where stronger optical carrier suppression improves the upconverted RF signal, weaker optical carrier suppression is preferred for an optical carrier transmitting two RF signals due to nonlinear distortion because the nonlinear distortion is reduced by using weaker optical carrier suppression. We find that the usable range of optical carrier suppression ratio is from 10 to 18 dB for RF signal upconverted to 20 GHz and beyond, and the best optical carrier suppression ratio is around 10 dB. We verify the concept and analysis with experiment. In experiment, we used two RFs at 6 and 18 GHz transmitting two 750 Mb/s signals. The experiment for the first time demonstrated that an optical carrier can transmit two RF signals using optical carrier suppression and showed that upconverted RF signals are degraded by nonlinear distortion, particularly for upconverted RF signal at 12 GHz, i.e. the RF signal at the lower frequency.

Composite broad-band fiber Raman amplifiers using incoherent pumping.

This report presents an investigation of composite fiber Raman amplifiers (i.e., a distributed fiber Raman amplifier followed by a discrete fiber Raman amplifier) with in-coherent pumping, compared to conventional coherent pumping. It is shown that a flatter gain, noise figure and optical signal-to-noise ratio (OSNR) over 100-nm bandwidth can be achieved simultaneously by using two counter-incoherent pumps, compared to using six counter-coherent pumps. Moreover, it is found that a further improvement in gain, noise figure and OSNR flatness can be obtained in composite fiber Raman amplifiers with bi-directional incoherent pumping. The flatness of both gain and OSNR with a ripple of 1 dB is predicted by using one co-incoherent pump and one counter-incoherent pump.

Noise statistics in optically preamplified differential phase-shift keying receivers with Mach-Zehnder interferometer demodulation.

We report an analysis of the noise statistics for an optically preamplified differential phase-shift keying (DPSK) receiver with balanced and single-port detections. It is found that identical signal-amplified spontaneous emission beating noise exists for bits 1 and 0 in DPSK balanced detection. It is also revealed that the bit error ratio (BER) of a DPSK receiver with balanced detection has no direct relation to the conventional Q factor. Moreover, an analytic BER expression for the DPSK balanced detection receiver is presented.